Nuria Marbà

Adjunct Faculty, Oceans and Coasts

Ph.D.

Nuria Marbà is a researcher with IMEDEA (CSIC) in Spain and works in the department of Global Change Research. Nuria has worked extensively on the population biology and ecology of seagrasses and is now particularly interested in how these ecosystems are responding to the effects of climate change.

Seagrass Herbivory Levels Sustain Site-Fidelity in a Remnant Dugong Population

Herds of dugong, a largely tropical marine megaherbivore, are known to undertake long-dis-
tance movements, sequentially overgrazing seagrass meadows in their path. Given their
drastic declines in many regions, it is unclear whether at lower densities, their grazing is
less intense, reducing their need to travel between meadows. We studied the effect of the
feeding behaviour of a small dugong population in the Andaman and Nicobar archipelago,
India to understand how small isolated populations graze seagrasses. In the seven years of
our observation, all recorded dugongs travelled either solitarily or in pairs, and their use of
seagrasses was limited to 8 meadows, some of which were persistently grazed. These
meadows were relatively large, contiguous and dominated by short-lived seagrasses spe-
cies. Dugongs consumed approximately 15% of meadow primary production, but there was
a large variation (3–40% of total meadow production) in consumption patterns between
meadows. The impact of herbivory was relatively high, with shoot densities c. 50% higher
inside herbivore exclosures than in areas exposed to repeated grazing. Our results indicate
that dugongs in the study area repeatedly graze the same meadows probably because the
proportion of primary production consumed reduces shoot density to levels that are still
above values that can trigger meadow abandonment. This ability of seagrasses to cope per-
haps explains the long-term site fidelity shown by individual dugongs in these meadows.
The fact that seagrass meadows in the archipelago are able to support dugong foraging
requirements allows us to clearly identify locations where this remnant population persists,
and where urgent management efforts can be directed.

1. Historical declines of marine megaherbivores have led to a view of seagrass communities structured largely by abiotic disturbance and plant competition. There is, however, growing recognition of the significance of top-down control through herbivory, on seagrass ecosystem processes, raising the question of how meadows functioned under historically high populations of megaherbivores. 2. We assess the impacts of such intense herbivory on seagrass meadow composition in the Lakshadweep islands (India), where high-density feeding aggregations of green turtles have persisted for over a decade. We use a series of complementary approaches: (i) natural herbivory exclosures (ii) published data on seagrass composition before and after turtles established (at one atoll: Agatti) and (iii) present species composition along a turtle herbivory gradient over multiple atolls.

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Journal Article

2013

Green turtle herbivory dominates the fate of seagrass primary production in the Lakshadweep islands (Indian Ocean)

Historical global declines of megaherbivores from marine ecosystems have hitherto contributed to an understanding of seagrass meadow production dominated by detrital path- ways — a paradigm increasingly being questioned by recent re-evaluations of the importance of herbivory. Recoveries in green turtle populations at some locations provide an ideal opportunity to examine effects of high megaherbivore densities on the fate of seagrass production. We conducted direct field measurements of aboveground herbivory and shoot elongation rates in 9 seagrass meadows across 3 atolls in the Lakshadweep Archipelago (India) representing a gradient of green turtle densities. Across all meadows, green turtles consumed an average of 60% of the total leaf growth. As expected, herbivory rates were positively related to turtle density and ranged from being almost absent in meadows with few turtles, to potentially overgrazed meadows (ca. 170% of leaf growth) where turtles were abundant. Turtle herbivory also substantially reduced shoot elongation rates. Simulated grazing through clipping experiments confirmed this trend: growth rates rapidly declined to almost half in clipped plots relative to control plots. At green turtle den- sities similar to historical estimates, herbivory not only dominated the fate of seagrass primary pro- duction but also drastically reduced production rates in grazed meadows. Intensive turtle grazing and associated movement could also modify rates of detrital cycling, leaf export and local carbon burial, with important consequences for the entire seascape.

Ecosystem modifiers have the ability to significantly alter the ecosystem they inhabit sometimes with serious consequences for their own populations. We evaluated the ability of green turtles (Chelonia mydas) to modify seagrass ecosystems by their foraging activity. This study was conducted in a seagrass-dominated lagoon in the Lakshadweep Islands, Indian Ocean, where a stable high-density congregation of green turtles is present. We determined a gradient of turtle density in the lagoon and measured the intensity of turtle herbivory across the gradient. We then measured the impact of increasing grazing on seagrass structural parameters, growth and flowering along th﻿is gradient. Our results indicate that turtles substantially change seagrass meadow structure (canopy height, shoot length, width and density), reduce flowering and can potentially even cause changes in the species composition of the meadow. We discuss the implications of these results for seagrass ecosystem function, green turtle movement and human attitudes. When conserving ecosystem modifiers like the green turtle, any management strategy needs to include a detailed knowledge of the roles these species play in the ecosystems they inhabit.

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